This invention relates to magnetorheological fluids and, in particular, to base liquids suitable for magnetorheological fluid formulations.
Magnetorheological (MR) fluids are substances that exhibit an ability to change their flow characteristics by several orders of magnitude and in response times on the order of milliseconds under the influence of an applied magnetic field. An analogous class of fluids are electrorheological (ER) fluids which exhibit a like ability to change their flow or rheological characteristics under the influence of an applied electric field. In both instances, these induced rheological changes are completely reversible. The utility of these materials is that suitably configured electromechanical actuators which use magnetorheological or electrorheological fluids can act as a rapidly responding active interface between computer-based sensing or controls and a desired mechanical output. With respect to automotive applications, such materials are seen as a useful working media in shock absorbers, for controllable suspension systems, vibration dampers in controllable powertrain and engine mounts, and in numerous electronically controlled force/torque transfer devices, such as clutches and brakes.
MR fluids are noncolloidal suspensions of finely divided (typically one to 100 microns in diameter) low coercivity, magnetizable particles of a material such as iron, nickel, cobalt, and their magnetic alloys dispersed in a base liquid or liquid vehicle such as a mineral oil, synthetic hydrocarbon, water, silicone oil, esterified fatty acid or other suitable organic liquid. MR fluids have an acceptably low viscosity in the absence of a magnetic field but display large increases in their dynamic yield stress when they are subjected to a magnetic field of, for example, about one Tesla. At the present state of development, MR fluids appear to offer significant advantages over ER fluids, particularly for automotive applications, because the MR fluids are less sensitive to common contaminants found in such environments, and they display greater differences in Theological properties in the presence of a modest applied field. Examples of magnetorheological fluids are illustrated, for example, in U.S. Pat. No. 4,992,190 issued Feb. 12, 1991, entitled xe2x80x9cFluid Responsive to a Magnetic Fieldxe2x80x9d; U.S. Pat. No. 5,167,850 issued Dec. 1, 1992, entitled xe2x80x9cFluid Responsive to a Magnetic Fieldxe2x80x9d; U.S. Pat. No. 5,354,488 issued Oct. 11, 1994, entitled xe2x80x9cFluid Responsive to a Magnetic Fieldxe2x80x9d; U.S. Pat. No. 5,382,373 issued Jan. 17, 1995, entitled xe2x80x9cMagnetorheological Particles Based on Alloy Particlesxe2x80x9d; and U.S. Pat. No. 5,667,715 issued Sep. 16, 1997, entitled xe2x80x9cMagnetorheological Fluids.xe2x80x9d
As suggested in the above patents and elsewhere, the viscosity of a typical MR fluid, in the absence of a magnetic field, is a function of variables such as base liquid composition, particle composition, particle size, the particle loading, temperature, and the like. However, in the presence of an applied magnetic field, the suspended magnetizable particles agglomerate to thicken or gel the MR fluid and drastically increase its effective viscosity. In the absence of a magnetic field, the base liquid must have an acceptable viscosity over a range of continuous operating temperatures. The viscosity of the MR fluid is acceptable if the base liquid is flowable at all temperatures within the range of continuous operating temperatures. For example, a suitable base liquid should have a viscosity in the range of about 13 centipoise (cp) to about 16 cp at a continuous operating temperature of about 20xc2x0 C., and a viscosity in the range of about 90 cp to about 120 cp at a continuous operating temperature of about xe2x88x9220xc2x0 C.
The base liquid must also exhibit compatibility with any elastomeric seals which the MR fluid wets in the MR device and which maintain the MR device liquid-tight. Furthermore, the base liquid must have a depressed volatility so that significant amounts of MR fluid are not vaporized or volatized. The elastomeric seals in MR devices are neither designed nor intended to provide a gas-tight fit. As a result, volatized base liquid can escape from the MR device by permeating between the elastomeric seals and their respective sealing surfaces. Finally, the base liquid must have a pour point that is less than the minimum continuous operating temperature. The pour point of the base liquid represents the lowest ambient temperature at which the MR device can operate.
MR devices utilized in certain automotive applications subject the MR fluid to continuous operating temperatures ranging between about xe2x88x9240xc2x0 C. and about 100xc2x0 C. Synthetic hydrocarbon base liquids currently used for such MR fluids typically contain a mixture of synthetic hydrocarbons known as polyalphaolefins or PAOs that are derived from the C10 monomer 1-decene, H2C:CH(CH2)7CH3. Dimer 1-decene polyalphaolefin has a 20-atom carbon chain length and is oligomerized from the monomer. Dimer 1-decene polyalphaolefin has an acceptable viscosity over the operating temperature range of such MR device applications. However, dimer 1-decene polyalphaolefin has an unacceptably high volatility if heated to a temperature near the upper end of the operating temperature range of the aforementioned MR devices. Trimer 1-decene polyalphaolefin is a 30-atom carbon chain length molecule formed by a oligomerization reaction from the monomer. Trimer 1-decene polyalphaolefin has a negligible volatility when heated to a temperature near the upper end of the operating temperature range but has an unacceptably high viscosity over the operating temperature range of such MR devices. To provide a base liquid with an acceptable volatility and viscosity and a volatility considered suitable for use in fluid formulations used in such MR devices, trimer 1-decene polyalphaolefin and dimer 1-decene polyalphaolefin are blended.
The viscosity and the volatility of these mixtures of dimer 1-decene polyalphaolefin and trimer 1-decene polyalphaolefin, such as a typical 50:50 blend of a mixture by volume, are superior in these MR device applications to a base liquid comprising either one of the 1-decene polyalphaolefins alone. The addition of dimer 1-decene polyalphaolefin to the blend lowers the effective viscosity of the mixture to an acceptable value. However, the significant volatility of the dimer 1-decene polyalphaolefin at temperatures less than about 100xc2x0 C. contributes to an increasingly significant loss of the base liquid of the MR fluid. Thus, an MR device containing an MR fluid formulated with a base liquid consisting of a 50:50 mixture of dimer 1-decene polyalphaolefin and trimer 1-decene polyalphaolefin would exhibit a significant loss of base liquid about its elastomeric seals when the MR device is operating at a continuous operating temperature near 100xc2x0 C.
In certain MR devices used in automotive applications for vibration damping, annular elastomeric seals are utilized to provide a dynamic seal for a piston rod attached to a piston which reciprocates in response to the applied vibrations. The inner and outer diameters of such annular elastomeric seals can be dimensioned so as to provide liquid-tight seals, respectively, with the exterior of the moving piston rod and with the gland or sealing groove in which the seal is captured. However, such sizing would result in a high friction between the piston rod and the elastomeric seal if the seal experiences a volumetric expansion when exposed to the MR fluid.
Seal swell is the swelling of elastomeric gaskets or seals as a result of exposure to petroleum, synthetic lubricants, or other hydraulic fluids. Elastomeric seal materials vary widely in their resistance to the effect of such fluids. To take advantage of the volumetric expansion due to seal swelling, the elastomeric seals in MR devices are intentionally undersized to minimize the friction between the piston rod and the seal and to provide a moderate amount of swelling which is relied upon to improve the sealing action. Specifically, these MR devices require that the elastomeric seals swell by about 2 percent to about 6 percent by volume, as quantified by a 70 hour soak in the particular MR fluid at 100xc2x0 C. On the other hand, excessive swelling of elastomeric seals in excess of about 6 percent is equally undesired since the performance of the MR device will be degraded. The seal swell provides effective sealing of the reservoir or damper body holding the MR fluid and participates in minimizing fluid loss from the MR device.
Polyalphaolefins cause elastomeric seals wetted by the MR fluid in an MR device to shrink or, at best, to swell an amount insufficient to prevent loss of the base liquid of the MR fluid about the seals. Therefore, a base liquid formed from a mixture of dimer 1-decene polyalphaolefin and trimer 1-decene polyalphaolefin cannot provide the requisite amount of seal swelling to meet MR device requirements. As a result, the loss of MR fluid about the elastomeric seals may be significant in those MR devices which utilize a mixture of dimer 1-decene polyalphaolefin and trimer 1-decene polyalphaolefin as a base liquid and may contribute to premature failure of such MR devices.
Seal swelling also provides the elastomeric seals in an MR device with the ability to repair incremental losses of the outer surface due to frictional wear as the piston rod reciprocates. As material is removed from the outer surface, swelling of the newly exposed portion of the surface can at least partially restore the local seal and prevent or reduce leakage. A base liquid, such as a mixture of dimer 1-decene polyalphaolefin and trimer 1-decene polyalphaolefin, does not exhibit this property because the amount of seal swelling of wetted elastomeric seals is insufficient. The inability to maintain a liquid-tight seal as the elastomeric seal erodes would contribute to an accelerated fluid loss from the damper reservoir.
Fluid loss occurs in virtually all MR devices utilized in automotive applications due to loss of volatized base liquid, inherent aspects of dynamic elastomeric seals and the effects of frictional wear. Ideally, the loss of base liquid should be insignificant over the lifetime of MR damper. Base liquids for MR fluid formulations that exhibit a significant volatility and/or an insufficient seal swelling ability, such as mixtures of dimer 1-decene polyalphaolefin and trimer 1-decene polyalphaolefin, can accelerate the loss of base liquid from the MR device and cause the MR device to fail before the projected lifetime.
Premature failure of an MR device due to fluid loss can arise from at least two mechanisms. In one mechanism, the loss of base liquid can cause the MR device to lose its ability to effectively absorb the vibrational energy imparted by the piston of the MR device and dampen the vibration. Second, the loss of base liquid can increase the concentration of magnetizable particles in the MR fluid. As a result, the viscosity of the MR fluid is increased and, in the presence of a magnetic field, the effective viscosity of the MR fluid is likewise increased. It follows that the vibration dampening response of the MR device will be degraded because the piston of the MR device will experience a large resistance to movement as it moves in the MR fluid thickened by fluid loss.
There is thus a need to develop a base liquid for an MR fluid formulation having an acceptable viscosity, a low pour point, a low volatility and a suitable amount of seal swelling so as to reduce, inhibit, or eliminate the loss of base liquid from an MR device while providing suitable lubrication over the range of temperatures in which the MR device is operating.
The present invention provides a magnetorheological fluid formulation for a magnetorheological device having an acceptable viscosity, a low pour point, a low volatility, and an effective elastomeric seal compatibility. The magnetorheological fluid formulation comprises a suspension of magnetizable particles dispersed in a base liquid that is a mixture of a dimer 1-dodecene polyalphaolefin and a diester. The diester is provided in an amount by volume, such as between about 10% and about 45% by volume relative to the total volume of the base liquid, which is effective to sufficiently swell the elastomeric seals of the magnetorheological device wetted by the magnetorheological fluid by a given percentage, such as between about 2 percent and about 6 percent.